Perception
Every organism needs to adapt itself to the changing conditions in its environment, and therefore to receive informations from it, through senses. Sense organs encompass a wide range of signals: touch is a simple perception of close mechanical stimuli, while hearing is a specialized form of touch that perceives the vibrations on long distance; olfaction and taste perceive the chemical makeup of near compounds; sight is the perception of electromagnetic waves (light), while electroception and magnetoception perceive directly electric and magnetic fields, respectively. Most organisms have one or two highly developed senses more prominent than others. Many small organisms that need to know only their immediate surroundings deal well with "contact senses" such as touch and taste, while larger animals that live in open environment will benefit more from "long-range senses" such as sight and hearing; many mammals have a well-developed olfaction, while birds (and humans) prefer sight; bats and cetaceans have come up with echolocation, a derived form of hearing; and so on. Mechanical senses 'Touch' Also see here Tactile senses (perception of mechanical stress) are the most simple and primitive, found in virtually every organism on Earth. They include the perception of pressure, vibrations (see hearing, below) and tension of body tissues. For example, a complex network of nerves allows a fly to adjust the shape of the wings and the frequency of flapping to counter irregular wind, while earthworms and many insects gain most of their sensorial informations from vibrations in the ground. Particularly, social insects such as ants and honeybees communicate mainly through touch. Fish have two specialized tactile organs: the Weber organ, a group of vertebrae appendages that detect water pressure through changes of shape in the swim bladder, and the lateral line (also present in some amphibians), a system of hair cells than runs along the fish' side perceiving water currents and vortices. The venus flytrap, a carnivorous plant, has extraordinarily sensitive hairs that cause the leaves to close when they detect an insect. While uiseful and versatile, touch detect only obkects in direct proximity, and it's useless at greater range; besides unicellular and plant-like organisms, it would be developed mostly in dark, noisiy and turbulent environments and probably by slow-moving or static organisms, especially in a very dense medium and without large predators. Burrowing organisms would be suited to evolve a good sense of touch, though it wouldn't likely be the main sense. A particular method of tactile communication, found in Xenology, could be found in organisms that can alter their skin texture in shifting corrugations and papillae, as octopi do. 'Hearing' Hearing is the perception of sound waves, that is, the vibrations trasmitted through matter (ground, water or air). Like other waves, they can be classified by intensity (the amount of energy carried by the wave) and frequency (the number of cyclical variations that occur per unit of time). They're far slower than light, and their speed increases with the medium density (343 m/s through air at 20°C and 1 atm, 1482 m/s through water at 20°C, 5960 m/s through steel). Frequency is measured in hertz (Hz), where one Hz means one cycle per second. Human beings can hear sounds between 12 and 20 000 Hz; sounds with a lower frequency are called infrasounds, those with a higher frequency ultrasounds. Dogs, mice, dolphins, bats, etc. can hear ultrasounds; as a rule of thumb, the smaller an animal is, the higher are the sounds it can hear (or produce). Soundwaves are collected by the tympanic membrane or eardrum. In mammals, vibrations are also trasmitted through three bones (the auditory ossicles: malleus, incus and stapes) towards the cochlea, where they activate ciliate cells connected to the auditory nerve. Snakes lack an eardrum, but they can perceive sounds with the vibration of quadrate bone, while elephants can communicate by receiving with their feet low-frequency sounds transmitted in the ground. Mammals also have a pinna or auricula, a funnel-shaped structures that collects and directs sounds; many can move their pinnae with specialized muscles to direct them towards the source of a sound. Owls have horn-like feathers and a dish-shaped face with the same function. The presence of two (or more) ears allows to detect the direction a sound came from by measuring the difference between the time when it hit the eardrums; an owl can perceive a difference of 0.0002 seconds. 'Echolocation' Also see here and here. An extremely sharp hearing allows to reconstruct three-dimension "images" of the environment, based only on the way solid objects reflect sound. It's less efficient than sight, since it requires the animal to emit sound on its own (unless a constant but not too loud source of sound is already there). It'd be more useful where light is not already available, though it has the advantages of showing the inside of opaque objects, and beyond corners; on the other hand, this makes pinpointing the source more confusing. The wavelength of the sound is roughly equal (lightly smaller) than the resolution of the echolocation, that is, the size of the smallest recognizable detail. Since sound wavelength is always much larger than light's, echolocation has always a lesser resolution than sight (the smallest detail is larger). This wavelength can be computed, in meters, as l = v/f, where v is the speed (m/s) and f the frequency (Hz). Given than speed, and thus wavelength, increase in denser fluids, echolocation is more precise in a less dense medium (pressure and temperature usually have a negligible impact, though). It's also more effective if the sound has a high frequency. However, denser fluids carry soundwaves faster and more faithfully: while these images are blurrier in liquids than in air, they're also recognisable at a greater distance. In fact, it has been especially developed in cetaceans (whales and dolphins): pulses of high-pitched sounds are emitted by air passing through the phonic lips, then they're reflected by the concave skull and modulated by the melon (a fatty organ full of oils and waxes), that acts as a lens for soundwaves. Echoed sound is received through the lower jaw; it has been hypothized than the slight asimmetry in the bottlenose dolphins's dentition helps to pinpoint the sound sources. Night bats use echolocation to hunt in total darkness, emitting short pulses up to 100 000 Hz. A simpler form has also appeared in cave-dwelling birds, the oilbird (Steatornis) and the swiftlet Aerodramus, tenrecs and the shrews Sorex and Blarina. Aquatic insects such as gyrinidae beetles use a particular form of two-dimensional echolocation, producing vibrations on the water surface; many scorpions do the same thing on sand, and spiders perceive the trapped preys through the vibrations on the web. 2-D hearing is much more persistent than 3-D hearing. To an echolocation organism, soft tissues would be almost transparent, especially in water (since they have roughly the same effect on sound), while hard parts (bones, teeth, etc.), air bubbles and hollow cavities would be clearly "visible" even from the outside. The echo could be fractioned into "colours" just as light, according to its frequency, but the Doppler effect would raise the frequency for incoming object and lower it for parting ones, while the equivalent effect for light occurs only on enormous speed and distance. Chemical senses 'Olfaction' Also see here. Olfaction, or smell, is the perception of highly dispersed chemicals found in a fluid. It's not a good way to carry informations on a long range - molecules don't travel at great speed, except in strong wind, they don't travel in a specific direction as waves do and don't move at all through a solid medium, and they disperse quickly at greater distance from the source. In compensation, they quickly fill a small space and diffuse beyond corners and through fissures. They have a tendence to linger in time, which can be both an asset and an impairment (as they leave durable traces, but they can confound later smells). Smell diffusion isn't costly: a microgram of chemicals can persist for hours or days over several square kilometers. Male silkworms (Bombyx mori) can recognise the smell of s female 20 km away, thanks to an array of 17 000 cilia on their antennae, and start actively searching for her when the concentration reaches just 14 000 molecules/cm3. Humans have about 5 cm2 of olfactive epithelium, covered in 5 millions of chemiosensitive cells. Rabbits have 100 millions, terriers 150 millions, and a german shepherd reaches 225 millions over a surface of nearly 170 cm2. Many different animals (insects such as ants, mammals such as rats and dogs) can effectively communicate by releasing or transporting chemicals; it has been estimated that osmic messages over 10 m, with a steady 14 km/h wind, can carry an infomation flux of 100 bit/s for each chemical, the rough equivalent of four 5-letter english words. Olfactive communication doesn't have a precise temporal sequence as speech does, and it lacks a spatial resolution (unless it's perceived by organs very far apart, or while moving); also, smells cannot be converted to electric signals as with light and soundwaves, therefore harming telecommunication. Still, it could evolve as the main form of communication inside hot, thin and possibly opaque atmospheres, which would distort images and weaken sounds, but would ease the travel of molecules. 'Taste' Taste is closely related to olfaction: many reptiles "smell" the air with their tongue, and so do dolphins in water, while all land vertebrates have choanae, apertures between the throat and the nasal cavity that allow the passage of air and olfactive chemicals. In fact, smell makes up part of the flavor of most foods, along with temperature, touch and chemesthesis, the taste-like perception of fatty acids and calcium, the tingling of carbon dioxide in carbonated drinks, the astringency of tannin in tea and unripe fruit, the coolness of menthol, the piquancy of pepper, etc. True taste is organized into five main groups:'' bitterness'', usually unpleasant (since it's associated with toxins) is given by many chemicals such as quinine;'' saltiness'' is produced by the presence of sodium ions (and sometimes related ions such as lithium and potassium); sourness is the perception of acidity, that is, the concentration of hydrogen ions; sweetness, generally pleasant as it's associated with energy-rich food, is produced by simple sugars, mainly glucose; umami (generally described as "maty" or "savory", found in cheese, soy sauce, tomatoes and many fermented foods) is mainly induced by monosodium glutamate (MSG). Saltiness and bitterness are identified by ionic channels; bitterness, sweetness and umami by protein receptors. In most vertebrates, taste receptors are concentrated on the tongue and on the oral mucous membrane, as its main function is to judge the quality of ingested food. Flies and butterflies have taste receptors on their feet, in order to taste the food on which they land, while octopi have them on their tentacles, and catfish are entirely covered in them. Electromagnetic senses 'Sight' Sight (or vision) is the perception of moving electromagnetic waves, i. e. light. While light cannot travel around corners or most solid matter, it's extremely fast (nearly 300 000 km/s in every medium) and it usually has a small wavelength that keeps the waves from diffracting in fissures. Most importantly, most environments will have abundant sunlight, so the organisms aren't forced to produce light from themselves as echolocating organisms produce sound (which would be expensive in energy and would reveal one's position to predators). In caves, deep ocean and the surface of planets with opaque atmosphere, though, the need for light production might arise again. Since sight is so useful, we should expect it to arise wherever there is available light: eyes are estimated to have evolved 40 times on Earth. Squids, chameleons, zebrafish etc. can communicate by changing their colour by expanding or narrowing pigment-bearing cells (chromatophores)They include black or brown melanophores (pigment eumelanin), red erythrophores (pigment carotenoids), orange or yellow xanthophores (pigment pteridine), blue-green or metallic iridophores (guanine crystals that diffract light), blue cyanophores (uknown pigments) and white leucophores (guanine crystals that reflect light).; many organisms, such as fireflies and bacteria hosted by several deep-sea fish can produce light. Optical communication, though, isn't as effective: complex body motion, colour change and light production are much more expensive than sound production or chemical diffusion. 'Eye structure' Being so widespread in the animal kingdom, the eyes exist in a great variety of shapes and functions. Mollusks and arthropods have each every type of eye listed below, and complex eye with one or more lenses have appeared in vertebrates, cephalopods, insects, crustaceans, annelids and cubozoans. *The simplest eye is called pit eye (also stemma or ocellus). Found in most animal phyla, it's a depression of the body surface, covered in photoreceptor cells, which allows to see the direction light comes from. Wasps, spiders and scorpions have additional ocelli on the top of their head along with the main eyes; rattlesnakes have infrared-sensitive ocelli (see below) on the muzzle to perceive the prey's body heat. The pinhole eye, found in the chambered nautilus, is a pit eye with a narrow aperture that grants a better sense of directionality. The produced images, though, are very blurry. *A spherical lensed eye improves the resolution by using a sphere of material with a higher refractive index that focuses the light on the photoreceptor cells, often held in place by muscles; it appeared once in copepods, once in annelids and at least six times in mollusks. Some copepods have a multiple lensed eye, with up to three lenses per eye. *''Refractive cornea eyes'' are found in vertebrates, spiders and some insect larvae; and light is directed by a refracting vitreous fluid (needed only out of water, since it has the same effect on light). A flattened lens corrects spherical aberration, but allows a clear vision only in a restricted field: animals that need a wide field-of-view have a biconvex lens with inhomogeneous refractive index. *''Reflector eyes'' focus light without a lens, by reflecting it with mirrors. Rotifers, copepods and flatworm have such eyes, though too small to produce sharp images; scallops have the edge of their shells lined by up to 100 reflector eyes. The only vertebrate that employ reflection is the spookfish, with two arrays of photoreceptors, one of which is located under a mirror of guanine crystals. *''Compound eyes'', common among arthropods, are composed by a large number of small, simple eyes (ommatidia); each ommatidium points in a different direction, granting a wide field-of-view and the effective detection of fast movements. Since the lenses are small, though, the images cannot be very clear; anyway, at a small size the lens of simple eyes couldn't have a better resolution. The simplest form of compound eye, the apposition eye, is found in every group of arthropods, and also annelids and bivalves. Typically, the lens focuses light on one side of a tube (rhabdom); each ommatidium forms one small image, which is combined with the others in the field-of-view. In superposition eyes, each ommatidium receives the whole image, combined by the brain with the other slightly different ones. The lens(es) can be made out of different materials: aragonite in chitons', calcite in trilobites and water-soluble crystallins in vertebrates. Other solid materials with a high refractive index include rock salt, silicon, diamond, amber and sucrose. The brittle star Ophiocoma wendtii is entirely covered in calcite crystals, connected to nerves, that make the entire body one great compound eye. Other adaptations can be included in the eye: fast-moving and predatory insects have a fovea, a region in their compound eyes with larger and flatter ommatidia, which allow for a better resolution; nocturnal hunters such as cats and abyssal squids have a tepetum lucidum, a layer of reflective guanine crystals that multiply the available light; the four-eyed fish (Anableps) has split eyes that allow it to see both up and down at the same time. Note: the resolution is given in a general sense (angular size, in minutes of arc, of the smallest recognisable detail) and then as the actual size of that detail at different distances. The resolution can be estimated through the Rayleigh criterion'' as q = 70l/A, where q is the resolution in degrees, l the wavelength of light and A the diameter of the ommatidium or pupil (both in the same unit of measure).'' 'Light and colour' Colour is the perception of the wavelength of light. Of the several bands of light that can reach the atmosphere of a planet, those with higher energy (gamma rays and x-rays), most ultraviolet, microwaves and far infrared are absorbed by gases (the same would happen in most likely atmospheres; also see here). This leaves three main bands: visible light (400-700 nm), near infrared (7-20 µm) and short radio waves (2 cm-10 m). Some organisms, such as bees and several birds, can see in near ultraviolet, which allows them to see otherwise invisible patterns (e.g. on flowers) but isn't very useful for navigation. The eye's light-sensitive layer (retina) contains two kinds of photoreceptor cells: rods perceive the intensity of light, and allow black-and-white vision, while cones perceive the colours. The human eye has three kinds of cones (trichromatic visions), with their pak of activity in the blue, green and red wavelengths. Primates such as human probably developed trichromatic vision to recognise ripe fruit, while most mammals are dichromatic; pinnipeds, cetaceans and owl monkeys are monochromatic. Zebrafish, mantis shrimps and many birds are instead quadrichromatic, with a fourth type of cone that allows them to see ultraviolet light; pigeons, butterflies and lampreys are suspected to be pentachromatic. Usually, nocturnal animals are less sensitive to colours. While infrared is about half of the radiation incoming from the Sun (more in K and M-class star, less in F-class stars), but since every object produces light in function of its temperature, the whole surface of a planet would shine in infrared light simply because of its thermal energy. The perception of infrared allows rattlesnakes to sense temperature differences of 0.002°C - useful to hunt warm-blooded preys in the darkness. While the largest wavelength produces blurrier images, using the Rayleigh criterion given above we can estimate that an eye with an aperture of A = 70l/q = 70*10-5/(1/60) = 4200*10-5 = 0.042 m = 4.2 cm is enough to perceive a wavelength of 10 µm with the same resolution of the human eye. The perception of infrared is essentially the perception of heat, which, like scents, can leave traces lingering in time: that makes it a very useful sense for hunters. As for radio waves, they're not as much useful. Stars don't irradiate a large amount of them, and their large wavelength disturbs vision: even for 1 cm waves, an eye 42 m wide is needed to see with human-like resolution. Even large organisms would need to cover their whole body with radio-sensitive cell to form any image. They could be found, perhaps, on cold planets with completely opaque atmospheres, where they'd see only a faint glow from the ground and maybe stronger ones from quasars and galactic cores. The amount of light entering in the eye can be regulated with light-absorbing pigments, as in Ophiocoma, or physically modifying the width of the eye's aperture (pupil); in many animals, such as cats, it changes shape, going from round to almond-shaped. Since light with a smaller wavelength is refracted more, a multifocal lens can be employed, whose parts have different refractive indices: in this case, the pupil is often a thin fissure, to leave uncovered only the right part of the lens (see here). Even thin pupils can have different shapes: in cetaceans it's a flattened U, in octopi a horizontal fissure, in sheep and goats a horizontal rectangle, in cuttlefish it's W-shaped. 'Depth perception' Depth perception is the ability of perceiving a three-dimension environment by measuring the relative distance of objects. There are different methods, such as the linear perspective (the apparent convergence of parallel lines at great distance), the aerial perspective (faraway objects appear blurrier due to the atmosphere) and the motion parallax (distant objects appear to move slower than closer ones). Many birds bob their head when walking to provide always motion parallax, while squirrels move sideways relatively to the observed object. The most effective method is stereopsis, which requires two eyes pointed at the same object: the image that reaches each eye is slighly different from the other, and by comparing them the brain, with a trigonometric operation, can estimate the distance from the objects. Predators, especially ambushers, usually have two frontal eyes to better perceive their distance from the prey, while herbivores have lateral eyes that grant them a wider field-of-view, sacrificing depth perception. Organisms that live in three-dimension environment, such as tree canopies, aso have frontal eyes (as in apes). Grassland grazers also tend to have eyes far from the snout tip to keep them always above the grass; in amphibian and semi-aquatic animals, such as hippos and crocodiles, eyes and nostrils are raised to emerge from water without a great exposition of the body (in the extinct hippo H. gorgops they were almost on stalks). While two eyes are thus much more useful than only one, there seems to be no advantage for three or more. In vertebrates, arthropods and cephalopods, complex image-forming eyes are only two, though smaller accessory eyes are known: the pineal eye found in many reptiles and amphibians (endowed with lens and retina in tuataras), or the three ocelli that many insects have on the top of their head (see image above). Clamworms have four eyes, while scallops have hundreds of simple reflector eyes, 'Polarized light' 'Electroception' 'Magnetoception' Other senses 'Thermoception' 'Equilibrioception' 'Proprioception' 'Interoceptions' 'Other' Nervous system 'Structure' 'Early nervous systems' 'Gangliar system' 'Encephalization and nervous hierarchy' 'Brain structure' 'Variations' References *Sensations on Xenology (also see following pages) Notes Category:Extraterrestrial life Category:Exobiology Category:Speculative physiology